1. Field of the Invention
This invention relates to the field of fan devices for augmenting air circulation through forced air air conditioning and heating systems and, more particularly, to such a device designed to fit externally over a forced air register.
2. Description of the Relevant Prior Art
Due to their inherent advantages, forced air central heating and cooling systems have become the dominating means of both residential and commercial climate control in the United States within recent years. Since such installations include a system of ducts for conducting forced air to individual rooms, such systems can be adapted to both centrally heat and cool a building. Typically, a central heating and/or cooling unit acclimatizes outside air to a comfortable temperature set by a thermostat and a central blower is used to circulate the tempered air throughout the ducts to heat or cool the rooms of the building.
Problems can arise in circulating the acclimatized air to more remote portions of the building, or to upper stories thereof. According to the laws of fluid mechanics and air flow, as the column of air to be moved lengthens, air flow measured in cubic feet per second becomes diminished. Thus, it may become necessary to provide an auxiliary air circulating fan in order to provide sufficient air flow to more remote parts of a building.
This problem is particularly acute when a forced air system is used to air condition a building having two or more stories. The air cooled by the central air conditioner is relatively heavy and hard to move. It is more difficult to overcome gravity and supply such cooled air to the upper stories of a building than it is to supply air warmed by a furnace. Hence, it is not uncommon for the second floor rooms of a house to remain uncomfortably warm during the hot summer months even though the central air conditioning unit is in use. One is forced to choose between lowering the thermostat to boost the output of the air conditioning unit, thereby cooling the lower story to an uncomfortable level and wasting energy or tolerating uncomfortably warm temperatures in the upper story rooms. In fact, it is not uncommon for the dwellers to supplement forced air central air conditioning by installing expensive individual air conditioners in second story bedrooms. Obviously, such a solution is wasteful and unsatisfactory.
It has long been known to boost air circulation from a forced air unit by installing auxiliary fans inside the ducts of the system. For example, U.S. Pat. No. 4,798,518 discloses such an auxiliary fan unit for use with the duct system of an air conditioning and ventilation system. The fan unit disclosed in the referenced patent has a freely turning radial impeller and associated drive motor. A guiding structure is provided downstream from the outlet of the impeller which directs the air flow coming radially from the impeller to an axial direction. In other words, this device redirects the air flow through an angle of approximately 90.degree..
Another example of an in-duct circulation booster is shown in U.S. Pat. No. 3,099,201. However, these devices and others like them suffer from obvious disadvantages. Since they must be installed in the ductwork, installation is cumbersome and difficult. Moreover, the unit must be exactly sized to fit inside the ductwork. Since these devices must be permanently installed, they cannot be moved from location to location as desired.
It is also known to provide an air circulation booster as an external unit to be fitted over the aperture of a forced air register. Examples of such devices are shown in U.S. Pat. Nos. 4,722,266 and 4,846,399. While these devices have the advantage of being portable and requiring no expensive installation, the relatively small and inefficient fans used in these devices limit their utility. The average second floor register exhibits about 0.6 inches of water back pressure. The weight of the column of air and the viscous drag of the air against the walls of the ductwork act to restrict air flow; and, if any air is to flow out of the register, it must experience a pressure differential greater than 0.6 inches of water. Hence, an external booster must be as efficient as possible to overcome this static back pressure. Furthermore, an external booster must efficiently and effectively couple to the air handling system. Also, it would be highly desirable that any external air flow booster be simple to install and remove.